Southern California Edison has spent the last year telling nuclear regulators and the public that major problems with its generators at San Onofre that leaked radiation last year were a complete surprise.But later today, the U.S. Nuclear Regulatory Commission (NRC) is expected to release a report that may radically change that picture.

According to two senior members of Congress, the report by generator manufacturer Mitsubishi shows that Edison knew about “serious problems” with the four generators before they were installed, yet “rejected enhanced safety measures” that could have helped prevent last January’s radiation leak.

If their description pans out, it would stand in stark contrast to Edison’s claims of being caught off guard by the leak, a result of extremely rapid damage to the generators’ tubes.

As recently as last month, Edison officials told the NRC there was no advanced warning of those problems when the $15-billion utility giant analyzed the generators’ safety risks.

“The adverse condition that later resulted in a tube leak was a deficiency associated with the design and was not known at the time the 50.59 evaluation was performed,” Edison wroteJan. 9in a letter to federal regulators.

The utility was responding to a formal request submitted to the NRC’s Petition Review Board from the environmental group Friends of the Earth seeking a rigorous license amendment process before Edison is allowed to restart the plant.

“It would appear that the [Mitsubishi] document referred to by Senator Boxer and Representative Markey provides proof that the [Edison] statement is untrue,” Friends of the Earth wrote in a statement this week.

Arnie Gundersen, a consultant to Friends of the Earth, has been saying for months that Edison misled the NRC about the changes.

Edison, meanwhile, strongly disputes that contention, saying in a company statement last week that it “would never, and did not, install steam generators that it believed would not perform safely.”

Mitsubishi’s report is at the center of an “expansive investigation” by the NRC, which can refer certain cases to the U.S. Department of Justice for criminal prosecution. The current probe, which apparently started in September, was publicly announced after an alarming letter last week from the lawmakers.

By opting not to go through a lengthy license amendment process for the replacement generators, Edison essentially affirmed that it expected no significantly greater risk of an accident from the equipment.

Severe consequences could follow if Edison is found to have misled the NRC. Making a “material false statement” to regulators is grounds for revoking the utility’s license to run the plant. It’s unclear whether the NRC has ever taken that step.

Edison, meanwhile, insists that it would never put the public’s safety at risk.

Earlier this week, Pete Dietrich, chief nuclear officer for Edison, posed that very question to himself publicly at a packed NRC hearing inside a Dana Point church, saying, “Is it our first priority, the safety of you and the people that live in these communities? I will tell you, at 2 o'clock in the morning and 2 o'clock in the afternoon, it absolutely is.”

But when reporters asked him for an interview just as the meeting adjourned, he hurried to a side-door exit and had security guards close the door behind him and confront the media.

To date, the Mitsubishi report has been kept secret because of NRC claims that it contained proprietary information.

Yet following a week of public pressure, the agency on Thursday gave Mitsubishi a Friday deadline to provide a redacted version of the report for public release.

That represents a major shift from the NRC’s position Tuesday night in Capistrano Beach, when a spokesman insisted there were no plans to make the document public.

The NRC investigation isn’t the only probe Edison faces in the aftermath of last year’s leak.

The California Public Utilities Commission is also considering whether electricity customers should continue paying more than $50 million per month for running the idled San Onofre.